Useful as a flux in the middle to high temperature ranges; in small amounts is very active, contributing to a smooth, even and trouble-free glaze. However, glazes which rely mainly on zinc as a flux have a tendency to crawl and may also be subject to pitting and pin holing. When used in large quantities it produces opacity, matt finishes and dryness of texture (5-15%). Zinc Oxide contributes greatly to opacity when used in conjunction with zirconium (1-4%). A material that can have a number of effects on the performance of a glaze. It is the only source of zinc for glazes and in small quantitiescan alleviate crazing problems and increases the firing range. In larger quantities it can be used to supplement opacifiers such as tin. Above 10% it develops a cool white, crystalline matt surface. It does not opacify those glazes containing boron.